This invention relates to amplifier circuits, and more particularly to a distortionless amplifier circuit having bipolar transistors.
In an amplifier circuit using bipolar transistors, the output signal is unavoidably distorted due to the nonlinearities of the transistors. The degree of distortion is usually reduced by negative feedback, but this is disadvantageous in that oscillation may be caused in the amplifier or the gain of the amplifier decreased. In order to overcome these drawbacks, an amplifier circuit has been proposed, which decreases the degree of signal distortion while operating stably, without using a negative feedback technique. In this known distortionless amplifier circuit, currents are supplied, in a predetermined ratio, to a pair of bipolar transistors which are of opposite electrical conductivity, so that the difference between the base-emitter voltages V.sub.BE of the transistors is maintained constant or zero and the nonlinearities of each transistor cancel one another.
FIG. 1 shows one example of such an amplifier circuit. A PNP transistor Q.sub.2 and an NPN transistor Q.sub.3 are amplifying transistors whose nonlinearities should be cancelled out. An input v.sub.i is applied to the base of the transistor Q.sub.2, the emitter output of which is applied to the base of the transistor Q.sub.3. In order to supply currents I.sub.2 and I.sub.3 with a predetermined ratio to the transistors Q.sub.2 and Q.sub.3, a current mirror circuit consisting of transistors Q.sub.5 and Q.sub.6 and emitter resistors R.sub.1 and R.sub.2 is provided. The transistor Q.sub.5 is diode-connected, so that a current defined by a current mirror ratio with respect to a current flowing in the transistor Q.sub.5 is also provided by the transistor Q.sub.6. The current mirror ratio is set by the resistors R.sub.1 and R.sub.2.
The circuit output is provided through an output load resistor R.sub.L. The transistor Q.sub.5 and an output current supplying transistor Q.sub.4 also form a current mirror circuit, so that a current I.sub.4 in a predetermined ratio with the current flowing in the transistor Q.sub.5 is supplied by the transistor Q.sub.4. This current ratio is defined by the ratio of the resistance values of resistors R.sub.1 and R.sub.3. In the circuit shown in FIG. 1, in order to make an output (V.sub.O) DC voltage zero, a constant current source (I.sub.4) is included for providing current which is substantially equal to the DC current I.sub.4 from the transistor Q.sub.4. Therefore, substantially no DC current is fed to the resistor R.sub.L. However, in practice, the output v.sub.O unavoidably involves a DC off-set voltage, and accordingly some DC negative feedback is required. For this purpose, an emitter current controlling transistor Q.sub.7 is connected between the emitter resistor R.sub.E of the transistor Q.sub.3 and a negative power source (- Vcc), and the transistor Q.sub.7 is controlled by a DC feedback system circuit 1, so that the output off-set voltage is zeroed.
A field-effect transistor Q.sub.1, resistors R.sub.g and R.sub.S and a constant current source (I.sub.D) form an input stage amplifier for providing a base input v.sub.i for the PNP transistor Q.sub.2.
With the circuitry arranged as shown in FIG. 1, the output v.sub.O can be represented by the following expression (1): EQU v.sub.O =(v.sub.i +V.sub.BE2 -V.sub.BE3).multidot.R.sub.L /R.sub.E ( 1)
where V.sub.BE2 and V.sub.BE3 are the base-emitter voltages of the transistors Q.sub.2 and Q.sub.3, and I.sub.3 =I.sub.4. In general, the relationship between the current I of a transistor and its base-emitter voltage V.sub.BE is represented by the following expression (2): ##EQU1## where k is Boltzman's constant, T is the junction absolute temperature, I.sub.S is the reverse saturation current, and q is the electron charge. Therefore, if the transistors Q.sub.2 and Q.sub.3 have equal electrical characteristics and are both satisfactory in thermal coupling, then ##EQU2##
If I.sub.2 /I.sub.3 =.alpha. (constant) is obtained by the current mirror circuit, then the expression (3) can be rewritten into the following expression (4): ##EQU3## That is, the expression (4) has a constant value .gamma.. Therefore, the following expression (5) can be obtained from the expression (1): EQU v.sub.O =(v.sub.i +.gamma.)R.sub.L /R.sub.E ( 5)
Thus, the output v.sub.O is a distortionless signal which is independent of the transistor base-emitter voltages V.sub.BE. If, in the expression (5), the current mirror ratio .alpha. is set to one (1), then .gamma.=0. Therefore, only a distortionless AC component is generated at the output V.sub.O.
However, since some DC off-set voltage is invariably developed in the output V.sub.O as was described before, it is necessary to employ some DC feedback to control the current through the transistor Q.sub.7, to thereby eliminate the DC off-set voltage. Thus, the equivalent emitter resistance r.sub.e of the transistor Q.sub.7 is connected in series with the emitter resistor R.sub.E which is a circuit gain determining resistor, and the value R.sub.E in the expression (5) can then be equivalently represented by (R.sub.E +r.sub.e). The resistance r.sub.e is non-linear, and therefore it is necessary to bypass it with a capacitor C.sub.E having a large capacitance, so that the emitter resistor is grounded out in an AC sense. Due to this capacitor, however, the frequency response is such that the gain in the low frequency range is different from the gain in the middle and high frequency ranges, and the phase is variable. Furthermore, depending on the materials of the capacitor C.sub.E, the tone quality is adversely affected. Since the capacitance is considerably large, it is impossible to provide the amplifier circuit in the form of an integrated circuit. In addition, since the emitter current of the transistor Q.sub.3 flows in the gain determining resistor R.sub.E, noise due to current is disadvantageously increased.